1. Field of the Invention
The present invention relates to a drive method for a plasma display panel, and a plasma display device used for a flat type television set and an information display, and specifically relates to a drive method for a plasma display panel, and a plasma display device for preventing a reversal of luminance when a total sustain cycle number decreases.
2. Description of the Related Art
A plasma display panel (PDP) is provided with a plurality of scan electrodes, and sustain electrodes extending in the horizontal direction, and a plurality of data electrodes extending in the vertical direction. Display cells are provided at individual intersections between the scan and sustain electrodes, and the data electrodes. In this specification, the vertical direction and the horizontal direction mean a vertical direction and a horizontal direction when the plasma display device is used while it is hanged on a wall, and respectively correspond to a column direction and a row direction in a drawing. FIG. 1 shows a schematic drawing showing a relationship among electrodes for the plasma display panel.
A number (a) of scan electrodes Sc1 to Sca, and a number (a) of sustain electrodes Su1 to Sua extending in the horizontal direction are provided alternately, and data electrodes D1 to Db extending in the vertical direction are provided in orthogonal to the scan electrodes and the sustain electrodes in the plasma display panel as shown in FIG. 1. Generally, the scan electrodes and the sustain electrodes are provided on a front substrate (not shown), and the data electrodes are provided on a rear substrate (not shown). A discharge space is formed between the front substrate and the rear substrate. The sustain electrodes Su1 to Sua may be connected together. One display cell 101 is placed at each of intersections between the scan and sustain electrode, and the data electrode. Therefore, when (a) of the scan electrodes, (a) of the sustain electrodes, and (b) of the data electrodes are provided in the plasma display, there exist total of (a×b) of the display cells 101. For three display cells 101 successive in the horizontal direction, any one of them emits red light (R), any one of them emits green light (G), and any one of them emits blue light (B). These three display cells constitute one pixel.
A gradation expression method called as a subfield method is generally adopted in a plasma display panel. In the subfield method, one field (one frame) is divided into a plurality of subfields having different weights, and the gradation is determined by which subfields are selected. The subfield comprises a preliminary discharge period (a priming period), a write period (an address period), a sustain period, and an erase period in many drive methods.
There is a control method called as PLE (Peak Luminance Enhancement). The PLE controls a sustain cycle number (a sustain pulse number) for the individual subfields for every frame according to an average peak level (APL), and reduces a power consumption while increasing peak luminance. For example, the average peak level is large for an image display of a snow-covered mountain, and is small for an image display of a night sky. For the image of the snow-covered mountain, there is no large effect on the human vision when background luminance is slightly large. On the other hand, for the image of the night sky, because most of the display area may have background luminance itself, when the background luminance is high, the contrast largely decreases compared with the case of the snow-covered mountain. Thus, the sustain cycle number per field is decreased when the average peak level is high, and the sustain cycle number per field is increased when the average peak level is low in the PLE control.
A drive method is disclosed for decreasing a write period to secure a long sustain period, and to increase the luminance (Japanese Patent Laid-Open Publication No. Hei. 11-24628). FIG. 2 is a timing chart for showing a drive method similar to the drive method disclosed in the publication described above.
In the drive method disclosed in the publication, for example, one field comprises eight subfields, and all scan electrodes are scanned for the four upper level subfields having larger weights. A scan similar to interlace display is conducted for the four lower level subfields having smaller weights. Namely, the scan pulses are applied only on the odd number scan electrodes (mth (m: odd number), (m+2)th, (m+4)th, . . . ) for an odd number field (nth (n: odd number) frame), and the scan pulses are applied only on the even number scan electrodes ((m+1)th, (m+3)th, (m+5)th, . . . ) for an even number field ((n+1)th frame) as shown in FIG. 2. As a result, because the even number scan electrodes are not scanned in the odd number fields, and the odd number scan electrodes are not scanned in the even number fields, the write period is reduced accordingly, and the reduced amount of the period can be assigned for the sustain period. Also, though a line flicker may be remarkable in the interlace display, because the interlace display is limited to the lower level subfields, the influence of the line flicker is small.
However, when the average peak level is high in the PLE control, the sustain cycle number may be 1 in the plurality of lower subfields, namely in the subfields with smaller weights. Table 1 shows a relationship between the gradation level and selected subfields when the average peak level is high in the PLE control where one field comprises four subfields SF1 to SF4, and 11 gradation levels are realized.
TABLE 1SubfieldSF1SF2SF3SF4Weight1248Cycle number1124LuminanceGradation00.84level1◯2.082◯2.083◯◯3.324◯2.965◯◯4.206◯◯4.207◯◯◯5.328◯4.689◯◯5.9210 ◯◯5.92
While Table 1 shows the eleven gradation levels, 16 gradation levels can be realized when one frame comprises four subfields.
When there are a plurality of subfields whose sustain cycle number is 1, there may be a reversal of the luminance, namely a case where a higher gradation level has lower luminance than a lower gradation level in two successive gradation levels. FIG. 3 is a chart showing a relationship between the gradation level and the luminance in Table 1. Because of the reversal of the luminance, there is a problem that a sufficient gradation expression is not realized.
When the method disclosed in Japanese Patent Laid-Open Publication No. Hei. 11-24628 is applied to the PLE control, because the sustain cycle number apparently increases, it is possible to prevent the reversal of the luminance. However, the interlace scan causes a change of an existing position of a displayed object within one filed, and the screen may momentarily become darker or brighter at a moment of a switching between a case where the write period is reduced and a case where the write period is not reduced, namely a switching between a frame where the lower four subfields are selected, and a frame where lower four subfields are not selected. As a result, the image quality degrades.